HOME SWITCHES
In industrial applications you will often find rugged switches like these. They are dust prove or even watertight (IP rating). Typically, these are limit switches, a machine part is physically touching them to trigger a response. The end of a travel distance would be one example.
For DIY- Projects there are several choices that can be determined by:
-
your budget
-
ease of wiring or
-
desired accuracy.
Category of switch by working principal:
-
Electro-Mechanical Switch
-
Inductive Sensor (metallic objects)
-
Capacitive Sensor (all objects)
-
Hall Sensor (magnetic)
-
Photoelectric Sensor
-
Just for completion I will include the pneumatic and ultrasonic sensor but will not further discuss these.
​
-
Use shielded wire, it will give you less headaches later on.
-
Find out what requirements your controller has, like voltage, NPN or PNP and how many open inputs do you have available.
-
Use a normally closed switch
TIPS FOR YOUR PROJECT
Have a look at the testing I do to find the right switch for my application.
My focus is to improve the repeatability of the home sequence of my machine especially for the dual driven Y-Axes
Here is what I like about it:
-
Its cost effective ($5)
-
It sports a NO & a NC contact
-
You can wire these in Series or Parallel if you have to.
-
With only 2 wires to connect it's easy to install.
-
Its small and can easily be fitted.
-
Surprisingly accurate
The Omron Electro-Mechanical Switch
The Inductive Prox-Switch
This type of proximity switch works on metals, but be aware that the sensing distance provided is for steel. You will need to cut that in half for aluminum.
What I like about the Prox-Switch:
-
There is a vast selection of different length, diameters and ratings available. So, most likely there is one for your project.
-
No mechanics to wear out or cause drift over time this results in a very long service life.
-
works even if some dirt (oil, coolant, dust...) collects on the sensor or the switch flag.
The Magnetic Proximity Sensor
Hall Sensor
​
I think that the Hall-Sensor is not an ideal choice as a home switch for a metal cutting CNC.
-
The magnet can pick up metal dust that will affect the sensing.
-
Best results are obtained when the magnet approaches the sensing surface head on. Not ideal, I think.
-
Also, in my testing, the approach speed made quite a difference as to where the axis stopped.
-
The magnet itself needs to be fastened and coolant can affect any glue. There are work arounds for this (plastic over molded magnets, clip in holders etc.), again just another concern.
The Photoelectric Sensor
What I like about the Panasonic Slot Sensor:
-
These have an excellent repeatability and are made for positioning.
-
Provides an NO and also NC output.
-
Very low current consumption of only 15mA and 5-24V this means you can potentially run this right of your control board.
-
There are different mounting options available, and it is small.
-
Good price (about $22).
Results
Not detectable
The chart shows the Total Span of 30 readings for each sensor.
There are different ways to look at this data, but the span will consider the worst scenario to expect from each sensor. I think it is not good to say my machine is accurate 86% of the time. This way you know within what limits the positioning will fall.
Metrol Precision Microswitch
What I like about the Metrol Precision Microswitch:
-
Hands down the most accurate switch with less than 0.001mm repeatability.
-
There are about 10 different styles available.
-
1-10 million cycle design with next to zero wear. ​
